According to U.S. Pat. No. 5,490,974, metal surfaces can be protected against metal-dusting corrosion by introducing from the outside a largely CO-free, gaseous or vaporous protective medium into a region between the metal surface and the thermal insulating layer.
Metal dusting corrosion occurs under the influence of a hot, CO-containing gas when the gas cools on the metal surface, and the CO is decomposed to form C+CO.sub.2 through the Boudouard reaction. Following decomposition of the CO on the metal surface, carbides are formed which lead to the destruction of the structure of the metal itself. Metal-dusting corrosion not only attacks steels, but also attacks nickel-based materials as well.
The intensity of the corrosive attack on the metal surface increases with increasing partial pressure of CO and with an increasing molar ratio between CO and CO.sub.2 on the metal surface. The phenomenon of metal-dusting corrosion has not yet been clarified scientifically in all details, but it is known that the potential for the formation of free carbon from the decomposition of CO plays an important role. The decomposition of CO takes place below the Boudouard equilibrium temperature, with this temperature limit being briefly referred to as the Boudouard temperature. The Boudouard temperature increases with increasing partial pressure of CO and with an increasing molar ratio between CO and CO.sub.2.
Plants components susceptible to metal-dusting corrosion are, for instance, lines or conduits through which flows synthesis gas, whose main components are CO and H.sub.2. Heat exchangers for cooling CO-containing gas, e.g. synthesis gas, must also be considered. The temperature on the metal surfaces susceptible to the metal dusting corrosion usually lie in the range of 300 to 1200.degree. C., and the metal surfaces are exposed to pressures of 1 to 150 bar. The hot gas, which causes the corrosion, mostly has a CO content in the range of 2 to 80 vol-%.